1. Field of the Invention
The present invention relates to the field of thermal gas sensors. More specifically, it relates to a thermal gas sensor for determining a concentration of gas in a two-component mixture at variable pressure. It also relates to a method for determining a concentration of gas in a two-component mixture of variable concentration.
2. Background
Thermal gas sensors take advantage of thermal conductivity properties of the gases to provide information on the nature of a gas or its concentration in a gaseous mixture. The thermal conductivity λ of a gas is its capacity to transport heat under the effect of a temperature gradient. It is an intrinsic magnitude of a gas at a given pressure and temperature and this is why its measurement is able to provide an indication of the composition of a gaseous mixture. Thermal gas sensors are used in particular for measuring the concentration of hydrogen (H2) in another gas such as oxygen (O2), nitrogen (N2), argon (Ar), carbon dioxide (CO2) or even air (assuming that it is of fixed composition), since hydrogen differs greatly from other gases because of its high thermal conductivity in comparison to that of heavier molecules. The following values are given for information purposes:
λhydrogen=0.84 Wm−1K−1
λair=0.012 Wm−1K−1.
Thermal gas sensors generally have an electrically insulating membrane of low thermal inertia, on which devices for heating the membrane and devices for measuring its temperature are arranged. The membrane is conventionally formed from a thin layer of silicon oxide or nitride deposited onto a silicon substrate, which is locally etched to the rear face of the membrane such that a gas flux can circulate on either side thereof. The heating devices and devices for measuring the temperature respectively comprise a first and a second electrical resistance formed by metal lines meandering over the front face of the membrane. The metal used for the temperature measuring devices has a variable resistance as a function of the temperature, such that measuring the voltage at its terminals enables the temperature of the membrane to be determined. When this latter is heated by the heating devices, its temperature rests at a stable value that is dependent on the thermal conductivity of the gaseous mixture or the ambient gas. As a result of this, the measurement of the temperature of the membrane provides an indication of the nature of the ambient gas or of the composition of the gaseous mixture. Reference is made to DE4228484 for more details on the structure and operation of such a gas sensor.
A gas sensor of the type described above allows measurement of a variable of a two-component gaseous mixture, i.e. the concentration of one of the gases, on the basis of one parameter: the temperature of a membrane in physical contact with said mixture. It assists in particular in determining the concentration of hydrogen in another gas, as explained above. There is considerable interest in determining the proportion of hydrogen in another gas with precision, since what is at stake in this case concerns the level of safety of installations and personnel. In fact, it is known that hydrogen forms a highly explosive mixture with oxygen, even at low concentrations. The same applies with air, which contains approximately twenty per cent oxygen. A thermal gas sensor provides such an indication at low cost and space requirement, hence its significant technical interest.
However, the measurement of a variable of a two-component gaseous mixture, in the case in point the concentration of one of the gases, on the basis of one parameter: the temperature of a membrane of low thermal inertia, is only possible if all the other variables of the mixture are constant. In particular, the pressure of the gaseous mixture has a significant influence on its conductivity. At variable pressure, it becomes impossible to determine the concentration of a gas in a two-component mixture on the basis of a single temperature measurement of the membrane.
Such a situation is encountered, for example, within an electrolyser unit. These devices are intended in particular for the production of gaseous hydrogen from water. These are currently the subject of significant developments since they offer a clean energy alternative to fossil fuels. Application EP 2 048 759, for example, describes a domestic installation for the production and storage of gaseous hydrogen using an electrolyser supplied with power by a photovoltaic installation. The hydrogen is then used as fuel in a fuel cell fitted, for example, in an electric vehicle.
In an electrolyser of the type described in application EP 2 048 759 hydrogen is produced from liquid water by means of an anode and a cathode. In a variant, the electrolyser is formed by an assembly of electrolytic cells, each of which having an anode and a cathode. Such a device is described in the patent document GB 1,145,751. Whatever the structure of the electrolyser, hydrogen is produced on the cathode side, while oxygen is produced on the anode side. The accumulation of these gases during the production process, respectively on the cathode side and anode side, causes the pressure within the electrolyser to increase progressively up to a value in the order of ten to several tens of bars. Because of the risks of explosion of an oxygen-hydrogen mixture, it is necessary to detect any presence of hydrogen in the oxygen and vice versa over the entire operating pressure range of the electrolyser. To achieve this and because of the above-mentioned limitation of thermal gas sensors, a pressure sensor is generally added to the gas sensor installed in the electrolyser. It is then possible to combine pressure and temperature measurements to get up to the concentration of gas to be determined. However, this solution increases the cost of this type of detection significantly, which represents a major disadvantage for a domestic installation.